Potassium ion channels (K+ channels) are ubiquitous transmembrane proteins that are major determinants of the membrane potential, i.e., the voltage difference that is present across plasma membranes, of almost all animal cells. In excitable cells, the K+ channels define the frequency and duration of action potentials, and play a fundamental role in neuronal integration, muscle contraction, and hormonal secretion. In nonexcitable cells, the K+ channels are pivotal to the maintenance of membrane potentials and the regulation of cell volume. These channels are thus important targets for the development of modulators that can be used to regulate fundamental cellular electrophysiology, particularly for use in therapeutic applications.